A critical event in regulated steroidogenesis is the acute response of steroidogenic cells to trophic hormones. This acute response predominantly reflects increased supply of cholesterol substrate to the inner mitochondrial matrix, where the cholesterol side-chain cleavage enzyme (P450scc) catalyzes the initial step in the biosynthesis of all physiological steroids. Recent analyses of patients with congenital lipoid adrenal hyperplasia (LCAH), an inherited disorder characterized by global defects in steroidogenesis, have established that the Steroidogenic Acute Regulatory protein (StAR) is essential for this delivery of cholesterol to P450scc. It thus becomes very important to understand the precise role that StAR plays in vivo and to delineate the mechanisms that regulate StAR expression. We have used targeted gene disruption to develop StAR knockout mice, providing a system in which the roles of StAR can be defined in a controlled setting within an intact endocrine milieu. Using these StAR knockout mice, cell lines will be derived from steroidogenic organs by transgenic expression of immortalizing oncoproteins. These cell lines will facilitate the study of StAR function, making is possible to express wild-type or mutated StAR protein within the optimal context of steroidogenic cells. Results with key StAR mutations that provide novel insights into its structure- function will be validated by transgenic reconstitution of StAR knockout mice. Finally, the mechanisms that regulate StAR expression will be evaluated in steroidogenic and non-steroidogenic cell lines, focusing on regulatory elements in the 5'-flanking region of the StAR gene that determine cell-selective and hormonally induced expression. Collectively, these studies will provide novel insights into the regulation and function of this essential component of the regulated biosynthesis of steroid hormones.